1. Field of the Invention
The present invention relates generally to methods and arrangements for detecting and responding to disturbances in electrical power systems and more particularly to methods and arrangements that detect various types of voltage disturbances while ignoring certain types of voltage disturbances and appropriately responding to the voltage disturbances.
2. Description of Related Art
Various types of voltage disturbances, such as sags, swells and transients, can occur on electrical power lines which are undesirable to continued proper operation of the various loads on the electrical system. Methods and arrangements have been proposed and/or used to detect and respond to these voltage disturbances including source-transfer systems which transfer the load to an alternate source, voltage regulators, so-called uninterruptable power supplies etc. In situations where the voltage disturbance is caused by a fault, it is undesirable to continue supplying the load where the fault is downstream of the source. However, where the fault is upstream and also in situations where the source sags for a prolonged period, it is desirable to transfer the load to an alternate source. Additionally, while various approaches are possible to reduce the magnitude and duration of transients, it is normally preferable to ignore these disturbances or respond to them in a manner different than other voltage disturbances. Arrangements to detect and respond to disturbances utilize various approaches including symmetrical components, DQ transforms, etc.
One arrangement to detect disturbances, shown in U.S. Pat. No. 4,002,950 to Dickerson, compares the actual voltage to a reference voltage and initiates action to interrupt supply to the load whenever the comparison exceeds a predetermined value.
While these arrangements may be useful and generally satisfactory for their intended purposes, they are not always capable of accurately and quickly detecting undesirable voltage disturbances which require action while accurately ignoring transients and the like. For example, the use of transforms results in relatively slow and desensitized responses, and are not very accurate in distinguishing between actual voltage sags/swells and transients, e.g. outrush current etc. Further, these arrangements are not capable of accurately predicting whether faults are upstream or downstream so that the continued supply of downstream faults is avoided.